Hereinafter, phases are expressed in radians. With increase of accuracy and variety of processing techniques, advanced measurement and analysis of a three-dimensional shape etc. of an object are sought after. Accordingly, various measurement methods have been developed. Among such measurement method, interference measurement techniques utilizing light interference, especially, digital holography makes it possible to obtain three-dimensional information of an object in a non-contact and non-destructive manner. Accordingly, digital holography is one of measurement methods which receive attention recently.
Digital holography is a technique for reproducing an image of a three-dimensional object by use of a computer on the basis of an interference pattern (interference fringe) which is obtained by light irradiation to the three-dimensional object. Specifically, for example, an image-capturing element such as a CCD (charge coupled device) is used to record an interference pattern formed by (i) an object light beam obtained by light irradiation to a three-dimensional object and (ii) a reference light beam which is coherent to the object light beam. On the basis of the interference pattern thus recorded, a computer carries out Fresnel transformation so as to reproduce an image of the three-dimensional object.
FIG. 26 is a schematic view illustrating an arrangement of a conventional digital holography apparatus (Non-patent Literature 1). A digital holography apparatus 120 has an optical system which includes a laser light source 101, a CCD camera 102, and a computer 110. A laser beam emitted from the laser light source 101 passes through a beam expander 103 and a collimator lens 104 so as to be a collimated laser beam. Then, the collimated laser beam is split by a beam splitter 105 into a reference light beam and an object light beam. The object light beam is reflected by a movable mirror 106 so as to be directed to a subject 111. Then, the object light beam is reflected by the subject 111 so as to reach an image-capturing plane of the CCD camera 102 via a half mirror 107. On the other hand, the reference light beam is reflected by a mirror 108, a PZT mirror 109, and the half mirror 107 so as to reach the image-capturing plane of the CCD camera 102. The CCD camera 102 records an interference pattern formed by the object light beam and the reference light beam which have reached the image-capturing plane. The computer 110 carries out calculation such as Fresnel transformation with respect to the interference pattern thus recorded, thereby obtaining a reconstructed image of the subject 111.
The subject 111 has a height h(x) along its depth direction from a position x (i.e., direction perpendicular to the image-capturing plane of the CCD camera 102).
In the digital holography apparatus 120, the reference light beam is incident upon the image-capturing plane of the CCD camera 102 almost perpendicularly. That is, the reference light beam and the object light beam are incident upon the image-capturing plane of the CCD camera 102 from substantially the same direction. It follows that a reconstructed image which is obtained by carrying out Fresnel transformation with respect to an interference pattern is made up of a zeroth-order diffraction image and a ±first-order diffraction image which are superimposed on each other. This makes it difficult to obtain a clear reconstructed image of the subject 111.
In view of this, there proposed a technique called phase-shifting technique (see Patent Literature 1 and Non-patent Literature 1). According to the phase-shifting technique, in order that a zeroth-order diffraction image and a ±first-order diffraction image are separated so that a high-accuracy reconstructed image is obtained, a phase of a reference light beam is shifted to a plurality of steps so as to obtain a plurality of interference patterns, and a desired reconstructed image is obtained from the plurality of interference patterns. Further, there proposed such a technique that a desired reconstructed image is obtained by use of a plurality of interference patterns among which a distance between a subject and an image-capturing element differs (Non-patent Literature 2). Hereinafter, the technique is referred to as optical path length-shifting technique.
The digital holography apparatus 120 carries out microscopic displacement of the PZT mirror 109 by use of a piezoelectric element so as to shift a phase of a reference light beam to three or four steps, and sequentially records respective interference patterns. Then, the digital holography apparatus 120 carries out calculation on the basis of a plurality of interference patterns thus recorded. Thus, the digital holography apparatus 120 can separately obtain a zeroth-order diffraction image and a ±first-order diffraction image.
At this stage, information indicative a position of the subject 111 along the depth direction which information is obtained on the basis of a phase distribution is folded into a wavelength range of the laser beam. In order to dissolve such folding so as to obtain information indicative of a position of the subject 111 along the depth direction which information originally goes beyond the wavelength range, it is necessary to carry out phase unwrapping. In a case where phase unwrapping is carried out by calculation based on positional information obtained from one interference pattern, a steep unevenness or the like of the subject 111 hinders correct phase unwrapping. As a result, obtained positional information is less-accurate one containing many errors. Accordingly, in order to obtain high-accuracy positional information, it is necessary to carry out phase unwrapping by the following optical technique.
The digital holography apparatus 120 changes an angle of the movable mirror 106 by Δθ/2 so as to record interference patterns between which an angle of a propagation direction of the object light beam incident upon the subject 111 is changed by Δθ. The use of such two interference patterns between which the angle of the propagation direction of the object light beam incident upon the subject 111 is changed by Δθ makes it possible to carry out correct phase unwrapping (Non-patent Literature 1).
Another optical technique is phase unwrapping utilizing two types of laser beams which differ in wavelength (Non-patent Literature 3). According to the technique, it is possible to freely change a synthetic wavelength, depending on how two wavelengths are combined. Further, it is possible to obtain a phase distribution equivalent to one which is obtained by use of a very long synthetic wavelength, as compared to a case of one wavelength. Accordingly, a phase folding is small. Further, increasing the number of wavelengths to be used makes it possible to expand a range of depth directions in which range phase unwrapping can be carried out.